The present invention relates generally to cold rolled steel strip, and more particularly to high strength, cold rolled steel strip having good resistance spot weldability and to methods for producing such steel strip.
Many of the components assembled into automobiles are formed from cold rolled steel strip, and these components may be subjected to resistance spot welding operations during their manufacture or during the assembly of the automobile. As a result of the current emphasis on decreasing the amount of gasoline consumed by automobiles, it is important that the weight of the automobile and its components be reduced because decreased gasoline consumption accompanies decreased weight. Heretofore, components of automobiles have been formed from ordinary, low carbon, cold rolled steel strip. This material, although having excellent resistance spot weldability, must be relatively thick in order to provide the strength required. Because ordinary cold rolled steel strip is relatively thick, the weight of the components manufactured from this strip, and of the automobile into which the components are assembled, is also relatively heavy.
The thickness of a steel strip can be reduced by increasing the strength of the steel. The strength of low carbon, cold rolled steel strip can be increased by adding small amounts (e.g., less than 1.0%) of alloying elements such as columbium, vanadium or titanium. Phosphorus can also be added to improve the strength of the steel. Vanadium, columbium and titanium increase the strength of the steel by forming precipitates in the matrix of the steel, while phosphorus increases the strength of the steel by a mechanism known as solid solution strength hardening.
Although all of these alloying ingredients increase the strength of the steel and thereby permit a reduction in thickness of the steel strip compared to a plain carbon steel strip of the same strength, each of these alloying elements, by itself, produces other drawbacks. For example, columbium, vanadium or titanium, besides being expensive, cause a loss of productivity during the rolling of steel containing these elements because such steels require reduced running speeds for the rolling mills used in their manufacture. These elements also tend to cause recrystallization problems in the steel and produce a non-uniform product when coils of steel strip containing these elements are subjected to a batch annealing operation which normally follows the cold rolling operation.
The problems described in the preceding paragraph do not occur when phosphorus is used as a strengthening ingredient. However, a high strength steel, the strength of which is improved by the addition of phosphorus, has relatively poor weldability. A steel strip has good resistance spot weldability when it is weldable over a relatively wide current range for relatively short weld times and when the weld nuggets produced on the steel strip exhibit what is known as a ductile peel test fracture which is substantially insensitive to increased hold time during the welding operation.
When columbium, or titanium alone, is used as a steel strengthening agent, the weldability of the steel strip is relatively good. However, in cold rolled steel strip strengthened with phosphorus, or phosphorus plus columbium, the range of currents at which these steels can be resistance welded at short weld times is relatively narrow so that appreciably longer weld times are required, compared to plain carbon steels, and the weld nuggets produced from the welding of such steels exhibit undesirable fracture characteristics in peel tests.